This invention relates generally to a cryogenic liquid storage tank, and more particularly concerns a cryogenic liquid storage tank having means for relieving the internal head pressure during filling in order to fill completely the cryogenic tank with cryogenic liquid.
Gases, having low boiling points at atmospheric pressures, such as carbon dioxide (CO.sub.2) and oxygen (O.sub.2), for example, present many difficulties and problems not encountered handling ordinary gases. In order to provide CO.sub.2 gas for use in fast food restaurants for carbonating soft drinks, for example, it has been necessary in the past to provide the compressed CO.sub.2 in single or clustered high pressure containers which are really best suited for customers with low consumption or sporadic use. Such service is expensive because of the high cost of handling the necessary heavy containers the weight of which is very high in comparison to the weight of the compressed gas contained therein.
Customers having high or moderately high demands for O.sub.2 or CO.sub.2 gas have also been serviced by means of high pressure tubular receivers installed on their premises. Such receivers are periodically serviced by means of a pump equipped liquid tank truck which transports the material to the customer's premises in cryogenic liquid form and charges such receivers with high pressure gas drawn from the vaporized cryogenic liquid in the tank truck. These tank trucks must be specifically equipped for this service and represent a large capital expense. Furthermore, the delivery of gas is time consuming because of the limiting capacity of the portable high pressure pumps.
Another way of storing such low boiling point gases, such as O.sub.2 and CO.sub.2, is to store them in a cryogenic storage tank in liquid form on the user's premises. Such a cryogenic liquid storage tank includes an inner vessel which holds the cryogenic liquid and an outer vessel within which the inner vessel is supported. There is an insulating space between the inner and outer vessels in which a vacuum is drawn and insulating material is positioned. Because of the low heat transfer from the ambient atmosphere outside of the outer vessel to the contents of the inner vessel, the liquid O.sub.2 or CO.sub.2 can remain in liquid form for some period of time before heat vaporization causes the vapor pressure of the O.sub.2 or CO.sub.2 to exceed a maximum pressure and to activate a regulator system for maintaining the vapor pressure within a safe range.
When such a cryogenic tank is installed on a customer's premises, such as a CO.sub.2 tank at a fast food restaurant, it is necessary periodically to refill the cryogenic tank with liquid CO.sub.2. The cryogenic CO.sub.2 tank is filled by means of a delivery truck carrying CO.sub.2 cryogenic liquid which makes its rounds from one customer to the next. In order to achieve the greatest efficiency, it is important to be able to fill the customer's tank as nearly full as possible without resorting to sophisticated high pressure pumps and/or regulator systems.
One way of filling a cryogenic tank on the customer's premises is to attach a single hose from a cryogenic tank on a transport truck to the inlet of the customer's cryogenic liquid storage tank. The vapor pressure in the transport truck's tank forces the liquid from the transport tank into the cryogenic tank on the customer's premises. As the liquid flows into the customer's cryogenic tank, the increase volume of liquid in the customer's cryogenic tank compresses the vapor above the liquid into a smaller and smaller space until the vapor pressure in the customer's tank exactly equals the vapor pressure in the transport tank. At that point, transfer from the transport tank to the customer's tank ceases even though the customer's tank may only be partially full.
In order to relieve the vapor pressure in the customer's cryogenic tank, the prior art suggests various ways of liquifying the CO.sub.2 vapor in the top of the customer's tank by means of eductors, J-shaped bubbler tubes, or J-shaped sprinkler tubes, all of which are shown in Remes et al., Ser. No. 448,729, filed Dec. 10, 1982 now abandoned.
Another way of assuring complete filling of the customer's cryogenic tank is disclosed in the applicant's prior patent, Gustafson U.S. Pat. No. 4,625,753, in which an automatic pressure relief means vents the CO.sub.2 vapor while the tank is being filled so that the tank may be completely filled. The automatic pressure relief means includes a cylindrical housing which is connected to the lower end of a vent tube. The housing has perforations to allow entry of cryogenic liquid. A buoyant float ball is enclosed within the housing. As the cryogenic tank is filled, the buoyant float ball floats upwardly into contact with an O-ring seat at the bottom of the vent tube thereby causing the pressure relief means automatically to close the vent tube. In an alternative embodiment of that invention, the automatic pressure relief means is combined with an eductor attached to the tank's inlet. The eductor entrains and condenses vapor within the tank to minimize the amount of vapor vented by the automatic pressure relief means.
While the automatic pressure relief means disclosed and claimed in Gustafson U.S. Pat. No. 4,625,753 performs the function of venting the tank so that the tank may be filled completely, the automatic pressure relief means uses a float ball which floats on the cryogenic liquid to close off the vent port once the cryogenic liquid has reached the vent tube at the top of the tank. In order to float in liquid CO.sub.2 or O.sub.2, the material for float ball is general confined to a plastic material. The use of a plastic material for the float ball limits the temperature in the tank the melting point of the plastic. During the manufacture of the vessel, the necessary welding operations cause the inside temperatures in the tank to exceed the melting points of available plastics. Consequently, the automatic pressure relief means with its plastic float ball can only be positioned inside the inner vessel after the inner vessel has cooled. Consequently, it is necessary that the automatic pressure relief means with its plastic float ball be attached to the outlet pipe by means of a threaded connector which can be installed after the welding operations have ended and the tank has had an opportunity to cool. Such an assembly constraint increases assembly time and results in a threaded connection that may be subject to failure.